The present invention relates to a closed or semi-closed breathing system for smoke diving and the like, comprising a pneumatically controlled breathing bag communicating in a circulation circuit with a breathing piece (e.g., mouthpiece or breathing mask) for application to the face of a user and with an absorption means for exhaled CO.sub.2, a pneumatic actuator arranged for alternating expansion and contraction of the breathing bag in accordance with the breathing pattern of the user, and a pressurized gas source coupled to the breathing bag to supplement the breathing gas therein.
There are known a number of embodiments of self-contained breathing systems. Breathing equipment for smoke diving (e.g., fire fighting operations wherein a fire fighter "dives" into an environment containing smoke and toxic gases) preferably is based on the supply of breathing gas through a breathing valve, whereas exhaled gas is "dumped" directly to the surroundings through a one-way valve ("open breathing system"). Alternative types of breathing equipment are based on recovery of exhaled gas in a "closed" or "semi-closed" circulation. This implies that exhaled gas--partly or completely--is purified of CO.sub.2 and supplied with oxygen so that it is again suitable as a breathing gas. With closed or semi-closed breathing systems there is achieved a long service life with a moderate gas supply, but they are normally difficult to breathe with because of the fact that the gas is recirculated by lung force. In comparison, good open breathing systems are easy to breathe with, but have a considerably shorter service life since one must be concerned with keeping the weight of the apparatus low. A substantial advantage with open breathing systems is that one is able to maintain a safety pressure (weak overpressure) in the breathing mask, so that the ingress of gases which are harmful to the user's health, is prevented.